In a typical cellular radio system, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a radio access network (RAN) to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network may also be broadcast in the cell. The radio base stations communicate over the radio interface operating on radio frequencies with the user equipments within range of the base stations.
In some versions of the RAN, several base stations are typically connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
A Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. Specifications for the Evolved Packet System (EPS) have completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to radio network controller (RNC) nodes. In general, in E-UTRAN/LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes, e.g., eNodeBs in LTE, and the core network. As such, the radio access network (RAN) of an EPS system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
In network assisted Device-to-Device (D2D) communications, user equipments and devices are allowed to use licensed spectrum resources to communicate directly with one another. As opposed to classical cellular communication methods, in which data is transmitted through an access point such as the radio base station, network assisted D2D communications uses a single link, also called a direct D2D link, between the two devices, thereby increasing the resource efficiency in the system.
Because of the single link usage, when two UEs or devices in the proximity of each other need to communicate using D2D technology, the devices must agree on a single set of resource, in the same frequency band, in which the D2D communication, i.e. transmission and reception of data, takes place. The resources dedicated to the D2D link may be assigned and managed by a cellular access point, e.g. eNB, as long as the communicating D2D pair belongs to the same operator and are registered at the same Public Land Mobile Network (PLMN).
Today, LTE cells are classified as suitable cells, for normal cellular services, or acceptable cells for limited services only, such as making an emergency call. Typically, cells belonging to another operator PLMN are classified as acceptable cells and the UE will keep searching for suitable cells to be able to obtain normal cellular services according to the subscription, UE capabilities and other aspects
Inter-operator D2D communications is problematic, since in D2D communication cases, D2D services may be provided by a peer device, e.g. UE-A, that possibly is served by a given PLMN, say PLMN-A. In this case, devices served by PLMN-B, e.g. UE-B, cannot access the services provided by such a device, because of the national roaming restrictions widely employed in many countries worldwide.
Thus, when the devices belong to, i.e. are served by, different PLMNs, respectively, within an overlapping geographical area, establishing the D2D link becomes problematic, since the two devices do not have access to the same set of spectrum resources. This problem is exacerbated by today's practice of forbidding national roaming within a country. For example, a device of a subscriber of Operator-A may not be allowed to roam into the PLMN of Operator-B within the same country. Therefore, D2D communications between devices that are served by different PLMNs within the same country may not be established.